Bumping gaia.json for 2 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/3c6792516c14
Author: Gareth Aye <gaye@mozilla.com>
Desc: Merge pull request #13672 from gaye/use-npm-mirror

Use npm mirror instead of npmjs.org

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https://hg.mozilla.org/integration/gaia-central/rev/15ac1f378958
Author: Gareth Aye <gaye@mozilla.com>
Desc: Use npm mirror instead of npmjs.org